Method for producing fine circuit lines and conductive board

ABSTRACT

The present invention provides a method for forming a circuit line which may be formed simply and economically by alleviating spread of an ink and exhibit excellent electric conductivity by having even height of the formed ink. The present invention further provides a conductive board with excellent high-dense electric conductivity including fine circuit lines. According to one embodiment of the present invention, a method for forming fine circuit lines comprises (a) treating at least one side of a circuit line pattern to be formed on a base substrate with an alkali metal hydroxide solution and (b) treating a hydrophobic ink in accordance to a circuit line pattern to be formed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Patent Application No. 2005-0062373 filed with the Korea Industrial Property Office on Jul. 11, 2005, the disclosure of which is incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to a method for forming fine circuit lines on a base substrate, particularly to a method for forming fine circuit lines by an ink-jet method.

2. Description of the Related Art

A conventional method for forming circuit lines on a base substrate is an etching resist treatment. But it is difficult to form fine circuit lines and requires a process of removing resist. Another method for forming fine circuit lines is a laser processing which after forming a water repellent coating layer all over a base substrate, a portion of the water repellent coating layer is removed by a laser light and a conductive ink is sprayed on the portion where the water repellent coating layer is removed. However, it requires an additional laser light producing device which increases a manufacturing cost and is complicated and inefficient since the water repellent coating layer has to be formed and then removed.

Therefore, an ink-jet method has been used these days to form fine circuit lines on a base substrate which allows several advantages including selective forming of fine circuit lines. However, it still needs to improve resolution which is very much affected by a diameter of an ink ejected from an ink-jet head, surface tension, and interfacial tension. Since metal nanoparticles are used to provide conductivity to the circuit lines, there is limitation to decrease a size of an ink-jet head and a diameter of droplets ejected. There is another problem of spread of an ink when the ink is ejected on a base substrate with a viscosity able to be ejected by an ink-jet method. Therefore, there have been various efforts to develop a method for forming fine circuit lines by an ink-jet method.

SUMMARY

As a solution to the forgoing problems of prior art, one aspect of the present invention provides a method for forming fine circuit lines simply and economically by alleviating spread of an ink with a pretreatment on a base substrate before ejecting an ink. Further, the present invention provides a high dense conductive board including such fine circuit lines.

Additional aspects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.

According to an embodiment of the present invention, a method for forming fine circuit lines comprises treating at least one side of a circuit line pattern on a base substrate with an alkali metal hydroxide solution and (b) treating a hydrophobic ink along with the circuit line pattern to be formed. Here, both sides of the circuit line pattern to be formed may be treated simultaneously in Step (a) and a treatment with a weak acid solution to the portion treated with the alkali metal hydroxide solution may be performed before Step (b). According to a preferred embodiment of the present invention, the weak acid solution is at least one selected from the group consisting of a formic acid and C1-C20 alkanoic acids

According to a preferred embodiment of the present invention, the alkali metal hydroxide solution is at least one selected from the group consisting of KOH, NaOH, and LiOH and it is preferable that its concentration be in the range of from 0.1 to 10M. The hydrophobic ink is tetradecanes.

The base substrate is a polyimide film and the alkali metal hydroxide solution and the hydrophobic ink are ejected by an ink-jet method.

According to another embodiment of the present invention, a conductive board including fine circuit lines prepared by the method for forming fine circuit lines is provided. It is preferable that the circuit lines have a width of 1 to 80 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing illustrating a fine circuit line according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION

Hereinafter, the method for forming fine circuit lines and the conductive board, according to embodiments of the present invention will be described in more detail with reference to the accompanying drawing.

FIG. 1 is a schematic drawing illustrating a fine circuit line according to a preferred embodiment of the present invention. Referring to FIG. 1, a high dense board is prepared by treating at least one side of a circuit line pattern to be formed with an alkali metal hydroxide solution in Step (A), reforming surface of a base substrate by reacting the alkali metal hydroxide solution and the base substrate in Step (B), treating a weak acid solution to the portion treated with the alkali metal hydroxide solution in Step (C) which is optional, ejecting a hydrophobic ink along with the circuit line pattern to be formed in Step (D), and curing a board including the formed circuit line in Step (E).

In Step (A), an alkali metal hydroxide solution 20 is treated to at least one side of a circuit line pattern on a base substrate 10. If surface of the base substrate 10 can be chemically reformed by reacting with the alkali metal hydroxide solution, it is not limited to any particular one, preferably a polymer substrate, more preferably a polyimide substrate. The polyimide substrate has been used as a base substrate of various flexible boards including PCB circuits and semiconductor integrated circuits. The base substrate may be a film type having a thickness of several μm or a sheet type having a thicker thickness of several mm. However, the polyimide substrate is not limited to such types or thicknesses in the present invention.

The treatment on at least one side of a circuit line pattern, preferably both sides, is to control the spread of an ink when the ink is ejected at a later process. A treatment part of the alkali metal hydroxide solution may eject the alkali metal hydroxide solution on the base substrate in constant intervals in accordance to the width of the circuit line. A degree of spread of the ink should be considered at this time. When the alkali metal hydroxide solution is ejected, a size of droplets ejected on the surface of the base substrate is 1.2 to 2 times of a size of droplets from an ink-jet head 30 and it is preferable as it is smaller within the range not to deteriorate a yield of a reaction for chemically reforming the surface. It is also preferable that portions treated with the alkali metal hydroxide solution be even and have constant intervals to achieve a continuous border. The narrower interval is required for the smaller size of droplets. This thus practically allows forming desired even circuit lines.

Both sides of the circuit line pattern may be treated with same alkali metal hydroxide solution or different from each other.

Here, the alkali metal hydroxide solution is prepared by dissolving at least one chosen from KOH, NaOH, and LiOH in water and adding ethylene glycol thereto. The ethylene glycol is added to control volatility, viscosity, surface tension, and degree of spread on the polyimide substrate.

It is preferable that the ethylene glycol use 0.1 to 1 volume ratio based to 1 of water, more preferable 0.3 volume ratio, since viscosity of the alkali metal hydroxide solution and degree of spread on the polyimide substrate become better. When the ethylene glycol is used more than 1 volume ratio, it deteriorates a reaction yield on the substrate surface. On the other hand, when it is used less than 0.1, the degree of spread of the alkali metal hydroxide solution on the substrate increases and thus it is not desirable.

It is preferable that a concentration of at least one hydroxide chosen from KOH, NaOH and LiOH be 0.1 to 10M in which it reforms the surface of the base substrate. When the concentration is less than 0.1 M, it is not proper to form an evenly continuous board. On the other hand, when it is higher than 10M, the solubility of the alkali metal hydroxide solution becomes lowered.

In the treatment process with the alkali metal hydroxide solution, temperature and humidity may be varied with reaction time, preferably 18 to 99° C. for 1 minute to 24 hours. When the temperature is lower than 18° C., the reaction takes too long while when it is higher than 100° C., it is difficult to control the reaction because water used as a solvent boils. It is more preferable to treat with 1 M of the alkali metal hydroxide solution at 50° C. for 5 minutes. Since an exothermic reaction is generally occurred on the surface of the polyimide substrate, it does not require to heat additionally. Such concentration of the alkali metal hydroxide solution and treating time also vary with a thickness of the polyimide substrate. It is preferable to treat for a short time at a mile temperature for a polyimide film having a thickness of several μm and it should not damage the polyimide film due to the matter of surface modification.

The alkali metal hydroxide solution may be treated by a mask method or ink-jet method but the ink-jet method is preferable to form fine circuit lines.

When the substrate is treated with the alkali metal hydroxide solution as in Step (B), the surface of the substrate is chemically reformed 23. For example, when the substrate is a polyimide, —N—CO— group of the surface is reformed to —NH and —COO^(—) groups by the alkali metal hydroxide solution and alkali metals of the hydroxide are adsorbed on the surface of the polyimide having carboxylic acids as positive ions of K⁺, Na⁺, or Li⁺. The surface of polyimide having carboxylic acids or carboxylic ions is reformed to be hydrophilic, so that it repulses against a hydrophobic solution.

In Step (C), a weak acid solution is treated to the portion treated with the alkali metal hydroxide solution which may be optionally performed to stabilize the carboxylic groups formed on the surface of polyimide in Step (B). When the portion treated with the alkali metal hydroxide solution is treated with the weak acid, positive ions of the alkali metal react with the weak acid, so that unnecessary metal ions are removed.

Examples of the weak acid include acetic acid, formic acid, phosphoric acid, carbonic acid and the like, more preferably at least one weak acid chosen from C1-C20 alkanoic acids such as formic acid and acetic acid having carboxylic groups. A concentration of this weak acid is in the range of form 0.01 to 5N. It is preferable that a polyimide substrate be treated with the weak acid at 18 to 50° C. for 1 minute to 24 hours. If it is treated at a temperature of less than 18° C., the reaction takes too slow and since the reaction is generally performed at a room temperature, it is not necessary to perform at a temperature of higher than 50° C. A reaction temperature and reaction time may be determined within an appropriate range which is enough to complete the reaction described above for the surface of the polyimide substrate. It is more preferable to perform with 1N of an alkanoic acid at 20° C. for 10 minutes.

A hydrophobic ink 50 is ejected in accordance to a circuit line pattern to be formed in Step (D). The surface of the polyimide substrate may be reformed to be hydrophilic by treating at least one side of the circuit line pattern to be formed before ejecting the ink. Use of weak hydrophilic ink is therefore more preferable in the present invention. Any kind of such weak hydrophilic inks, which are hydrophobic inks, may be used but inks should include metal nanoparticles since they are for forming conductive circuit lines. The hydrophobic ink may be a conductive ink using a solvent such as tetradecane, hexane, octane, toluene, 2-butoxyethyl acetate, acryl monomer, and epoxy monomer.

Since an ink containing a tetradecane solvent has a high boiling temperature, it dose not cause blocking of a head during ejection and thus allows to be used for a long period of time. Even if it has advantages, it has high spread on the surface of the substrate, so that it becomes to the difficult in practical use. However, a method for forming circuit lines according to the present invention allows forming fine circuit lines by controlling such spread of the ink. Further, when side of a circuit line pattern to be formed is modified by the above method, the spread of the ink is improved to have an even height of the ejected ink. Since the even height of the ink highly influences the conductivity of the conductive circuit line, the conductivity of the circuit line can be guaranteed with a particular even height of the ink. The method for forming circuit lines according to the present invention not only allows forming fine circuit lines but also improves the conductivity of the circuit lines. A size of droplets of the ink printed on the base substrate is generally 1.2 to 2 times to a size of a nozzle used. In the present invention, a size of droplets of the ink printed on the base substrate is not higher than 1.2 times which is smaller than that of conventional ones since a contact angle where the droplet of the ink contacts with the surface of the base substrate is reduced.

The board including such formed circuit lines is then cured in Step (E) to produce a desired high dense board. A reaction solution or any organic compound within the ink are removed and the remained metal which has the conductivity is

by curing in this step. The curing may be performed at 80 to 350° C. for 1 minute to 5 hours and it is preferable that since a polyimide is not stable at a high temperature, it is cured for a short time. According to a preferred embodiment, when tetradecane is used as a solvent, it is preferable to cure at 70° C. for 20 minutes. The curing may be also performed under nitrogen or argon at a lowered temperature. Further, formic acid may be added at this step.

A board including such formed conductive circuit lines 53 may be laminated if necessary, surface is treated by solder resist, symbol mark printing or HASL method, and terminal coating and holes or exterior fabrication is additionally performed to produce a desired conductive board.

The conductive board of the present invention may be used as single layer or multi-layer PCBs, LTCCs, MLCs, and the like but the board having circuit lines formed by the method described above may not be limited to its use. The circuit lines of the present invention has 1 to 80 μm of a width, so that a high dense circuit line board may be produced.

Embodiments relating methods of forming fine circuit lines were set forth above, and hereinafter, explanations will be given in greater detail with reference to specific examples.

EXAMPLE 1

After dissolving solid KOH 57 g to water 730 Ml, ethylene glycol 230 Ml was added to obtain a KOH solution. Both sides of a circuit line to be formed on a polyimide film was treated with the prepared KOH solution by an ink-jet method. A gap between heads was controlled to be 80 μm by using an ink-jet head having a nozzle size of 30 μm. After 5 minutes since the surface of the polyimide substrate had been treated with a KOH solution, 5N acetic acid was treated on the portion where the KOH solution was treated and left for 10 minutes. Tetradecane ink was ejected between reformed portions on the polyimide film by using an ink-jet head having a nozzle size of 30 μm. Such a formed board was then cured at 70° C. for 20 minutes to provide a board having an average circuit line width of 30 μm of which low electric resistivity was 3.1 μΩ·cm.

EXAMPLE 2

After dissolving solid KOH 41 g to water 730 Ml, ethylene glycol 230 Ml was added to obtain a KOH solution. Both sides of a circuit line to be formed on a polyimide film was treated with the prepared KOH solution by an ink-jet method. A gap between heads was controlled to be 80 μm by using an ink-jet head having a nozzle size of 30 μm. After 5 minutes since the surface of the polyimide substrate had been treated with a KOH solution, 5N acetic acid was treated on the portion where the KOH solution was treated and left for 10 minutes. Tetradecane ink was ejected between reformed portions on the polyimide film by using an ink-jet head having a nozzle size of 30 μm. Such a formed board was then cured at 70° C. for 20 minutes to provide a board having an average circuit line width of 30 μm of which low electric resistivity was 2.9 μΩ·cm.

Thetradecane ink was ejected by an ink-jet head having a nozzle size of 30 μm as in Example 1 and the formed board was then cured at 70° C. for 20 minutes to provide a board having an average circuit line width of higher than 40 μm. It was noted that when circuit lines was formed according to the present invention, fine circuit lines may be formed.

The present invention provides a method for forming fine circuit lines at a low cost and by a simple process by alleviating the spread of an ink with performing a pretreatment on the base substrate before ejecting the ink. The fine circuit lines produced by this method have even height and thus provide excellent electrical conductivity. Further it has an advantage of using an ink containing tetradecanes which have had limitation in their uses to the conventional methods.

Further, the present invention provides a conductive board having such fine circuit lines which exhibits high density and excellent electrical conductivity.

Although a few embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the present invention, the scope of which is defined in the appended claims and their equivalents. 

1. A method for forming fine circuit lines comprising: (a) treating at least one side of a circuit line pattern to be formed on a base substrate with an alkali metal hydroxide solution; and (b) treating with an hydrophobic ink along with the circuit line pattern to be formed.
 2. The method for forming fine circuit lines of claim 1, wherein both sides of the circuit line pattern to be formed are simultaneously treated in the step (a).
 3. The method for forming fine circuit lines of claim 2, wherein a step for treating the portion treated with the alkali metal hydroxide solution further with a weak acid solution prior to the step (b).
 4. The method for forming fine circuit lines of claim 3, wherein the weak acid is at least one selected from the group consisting of formic acid and C1-C20 alkanoic acids.
 5. The method for forming fine circuit lines of claim 1, wherein the alkali metal hydroxide solution is at least one selected from the group consisting of KOH, NaOH, and LiOH.
 6. The method for forming fine circuit lines of claim 5, wherein a concentration of the alkali metal hydroxide solution is in the range of from 0.1 to 10M.
 7. The method for forming fine circuit lines of claim 1, wherein the hydrophobic ink is an ink including tetradecane.
 8. The method for forming fine circuit lines of claim 1, wherein the base substrate is a polyimide film.
 9. The method for forming fine circuit lines of claim 1, wherein the alkali metal hydroxide solution or the hydrophobic ink is treated by an ink-jet method.
 10. A conductive board including circuit lines formed by a method for forming fine circuit lines comprising: (a) treating at least one side of a circuit line pattern to be formed on a base substrate with an alkali metal hydroxide solution; and (b) treating with an hydrophobic ink along with the circuit line pattern to be formed.
 11. The conductive board of claim 10, wherein the circuit lines have a width of 1 to 80 μm. 